E.g. a team adjacent to mine years ago had a dev who made a one-character typo in a commit that went to production. Which caused many $MM to incorrectly flow out the door post-haste. The bad transactions were fortunately reversible with some work, but I was floored that there were no automated tests gating these changes. It wasn't a subtle problem. The most basic, boring integration test of "run a set of X transactions, check the expected sum" would have prevented that failure.